The present invention relates to a method for the preparation of a cured rubbery product of an organopolysiloxane or, more particularly, to a method for the preparation of a cured rubbery product by irradiating an organopolysiloxane with ultraviolet light.
As is well known, cured products of silicones having rubbery elasticity are very useful materials in a wide field of applications. Curing of organopolysiloxanes by crosslink formation to be imparted with rubbery elasticity is carried out in several procedures according to the curability of the individual organopolysiloxanes. Most typically, silicone rubbers are obtained by heating a diorganopolysiloxane composition admixed with a curing agent such as an organic peroxide. Several other types of organopolysiloxanes can be crosslinked by an addition reaction or condensation reaction between functional groups bonded to the silicon atoms in the presence of a suitable crosslinking catalyst. Most of the organopolysiloxanes capable of giving rubbery products can react only by heating or in the presence of a foreign material such as the curing agent or crosslinking catalyst.
These curing conditions in the prior art bring about limitations to the application of rubbery products of silicones. For example, a heat-curable organopolysiloxane cannot be used together with a substrate material unless the substrate material has a sufficient heat stability in the curing condition of the organopolysiloxane. Also, organopolysiloxanes curable only in the presence of a curing agent or a crosslinking catalyst are undesirable in applications where the presence of such a foreign material or decomposition product thereof is detrimental depending on its nature.
One way to avoid the above described difficulties in curing organopolysiloxanes is to use radiative energy to effect curing. For example, there have been proposed various types of organopolysiloxanes or organopolysiloxane compositions curable by irradiation with light which is rich in ultraviolet. The compounds include those organopolysiloxanes which have vinyl groups bonded to the silicon atoms. Unfortunately, there have been known hitherto no vinyl-containing organopolysiloxanes curable at a sufficiently rapid rate when irradiated with ultraviolet radiation. Therefore, all of the hitherto known means for curing vinyl-containing organopolysiloxanes utilize other components such as a photosensitizer or another kind of an organopolysiloxane having functional groups other than vinyl groups.
U.S. Pat. No. 3,726,710 discloses a vinyl-containing organopolysiloxane which is cured when irradiated with strong ultraviolet light in the presence of a photosensitizer which is admixed therewith. This method, which relies on a photosensitizer, is not always satisfactory due to the rather poor compatibility of the photosensitizer with the organopolysiloxane. Moreover, although such a vinyl-containing organopolysiloxane admixed with a photosensitizer may be satisfactorily cured when irradiated as a thin film such as a coating layer on a release paper, when a layer of, for example, 0.5 mm or larger thickness is used, the curing is not satisfactory. The resulting product, because of unsatisfactory curing, lacks good mechanical properties and transparency.
The photocuring of vinyl-containing organopolysiloxanes by the reaction with the other types of functional group-containing organopolysiloxanes is also known, U.S. Pat. No. 3,873,499 discloses a composition comprising a mercapto-containing organopolysiloxane, a methylvinylpolysiloxane a gelation inhibitor while and U.S. Pat. No. 4,064,027 discloses a composition comprising a vinyl-containing organopolysiloxane and, an organohydrogenpolysiloxane having at least one hydrogen atom directly bonded to the silicon atom in a molecule and a photosensitizer. The curing in the former composition utilizes a crosslinking reaction between the vinyl groups and the mercapto groups in the organopolysiloxane and curing in the latter compositions utilizes an addition reaction between the vinyl groups and the silicon-bonded hydrogen atoms in the organopolysiloxanes.
The above discussed photocrosslinking reaction between the vinyl groups and mercapto groups proceeds with a satisfactorily high reaction rate only in the presence of a photosensitizer. The composition formulated with such a photosensitizer, however, has poor stability even when kept in the dark so that adequate storability requires the use of a stabilizing agent. Furthermore, a mercapto-containing organopolysiloxane is rather expensive due to the complicated steps synthesis thereof. Also, care must be taken in handling such an organopolysiloxane due to the unpleasant odor associated with the mercapto groups. The photocured products obtained from such a mercapto-containing organopolysiloxane are not desirable that they lack in stability have and inferior thermal resistance, particularly when the cured product is relatively thick.
Addition of a photosensitizer is also indispensable in the photocurable compositions disclosed in U.S. Pat. No. 4,064,027. The photosensitizer is undesirable because it lowers the mechanical strength those photocured products which have a relatively large thickness and because it causes the composition to foam.
The above described problems of photocuring of a vinyl-containing organopolysiloxane are partly due to the absorption characteristics of the organopolysiloxane in the ultraviolet region. As is known, vinyl groups bonded to the silicon atoms have strong absorption in the far ultraviolet region such that the extinction coefficient at the absorption maximum of 178 nm is as large as 17,000 but the vinyl-containing organopolysiloxanes are relatively transparent in the longer wavelength region while the energy distribution of the ultraviolet light emitted from conventional ultraviolet lamps is biased to the longer wavelength region and no sufficient energy output is obtained in the far ultraviolet necessitating the use of a photosensitizer. Thus, no satisfactory method is known hitherto for photocuring a vinyl-containing organopolysiloxane as such.
The search has continued for new methods of preparing cured products by the photocuring of a vinyl-containing organopolysiloxane. This invention was made as a result of that search.